To make seasoning: With the machine running, drop each ingredient, one at a time, into feed tube of a food processor and process, turning the machine on and off several times, until the seasonings are evenly chopped into to a coarse powder.

Heat the oil in a heavy casserole or a Dutch oven with a lid over medium-low heat until hot. Add the seasoning and onion, and cook, stirring, about 3 minutes, or until the onion is tender and seasonings are fragrant.

To make sauce: Combine all ingredients in a bowl. Add sauce to the onion mixture and bring to a boil. Stir in the potatoes so they're coated with sauce. Return the mixture to a boil, reduce heat to low, then cover and cook 20 to 25 minutes, or until the potatoes are tender. Scoop the sweet potatoes and sauce into a serving bowl. Serve with steamed rice, if desired.